Control and cut-off devices of this kind known up to now have been found to be unreliable and have not provided the desired function owing to a too primitive design, as unauthorized persons have been able to change the pre-set position without difficulty. Also the precision has been inferior and the valves have been incapable of function even after a short period of use. They have also been too unwieldy or costly in respect of manufacture and maintenance and have often consisted of two individual valves which have been built together.